Abstract Premature birth and bronchopulmonary dysplasia (BPD) are associated with recurrent wheezing, asthma and increased healthcare use following infection with rhinovirus (RV) and other common childhood respiratory vi- ruses. Survivors of preterm birth show persistent airway inflammation and vulnerability to respiratory viral infec- tion. Based on the paucity of airway structural abnormalities in ?new?, post-surfactant BPD and the lack of as- sociation with atopy, the mechanisms underlying increased respiratory symptoms and asthma development in former premature infants remain unknown. To explain this knowledge gap, we propose the novel hypothesis that in immature infants, exposure to supraphysiologic oxygen levels ?primes? the pulmonary innate immune system by partially activating lung CD103+ dendritic cells (DCs), leading to enhanced airway inflammation in response to future viral infections. Enhanced lung inflammation then leads to increased respiratory morbidity and airway hyperresponsiveness. Our central premise is that in the lungs of hyperoxia-exposed immature in- fants, binding of exposed necrotic cell cytoskeletal F-actin to its receptor Clec9a, serves as a priming mecha- nism for CD103+ DCs, leading to exaggerated pro-inflammatory responses to future TLR3 stimulation. We pro- pose the following Specific Aims: Specific Aim 1. Determine the mechanistic processes by which neona- tal hyperoxic exposure modulates lung CD103+ DCs functional properties. We will examine the effects of hyperoxic exposure on lung CD103+ DC function, measured by expression profile, phagocytosis and antigen presentation using the OT-I and OT-II systems. Functional effects of hyperoxia-exposed necrotic alveolar epi- thelial cells alone or with TLR3 stimulation on CD103+ DC activation will be assessed. Specific Aim 2. Deter- mine the role of lung CD103+ DCs in the exaggerated inflammatory response to RV infection and TLR3 agonism induced by hyperoxia. Two-day-old mice will be exposed to hyperoxia (0.75 FiO2) for 14 days and subsequently inoculated with RV or poly(I:C). The requirement of CD103+ DCs will be tested by in vivo deple- tion of CD103+ DCs and Batf3-/- mice, lacking lung CD103+ DCs. Sufficiency will be tested by: 1) adoptive transfer of CD103+ DCs from hyperoxia-exposed to air-exposed mice followed by RV or poly(I:C) treatment; and 2) transfer of CD103+ DCs from air-exposed wild-type mice into Batf3-/- mice, followed by hyperoxic expo- sure and RV or poly(I:C) treatment. Specific Aim 3. Define the contribution of Clec9a and its ligand F-ac- tin to the hyperoxia-induced enhanced inflammatory response to RV and TLR3 agonists. The require- ment of Clec9a for hyperoxia-induced lung CD103+ DC IL-12 production and enhanced inflammatory re- sponses to poly(I:C) and RV will be examined in Clec9a KO mice. Finally, we will correlate premature infant tracheal aspirate levels of Clec9a, F-actin and gelsolin, an F-actin severing protein, with IL-12, IFN-? and TNF- ? expression. Completion of the proposed studies will uncover novel innate immune pathways responsible for long-term respiratory morbidity in prematurely born infants and identify new targets for therapeutic intervention.